Radiation therapy of tumors in the lung and abdomen is currently limited by the inability to follow the motion of the tumor during the treatment. Magnetic resonance imaging has the potential to provide good images of the tumor, fast enough to allow imaging during therapy treatment. This would allow higher dose deposition in the tumor and spare the surrounding tissue. However, joining a Linac (therapy producing system) and an MR scanner is very challenging since the Linac uses electron acceleration to produce high-energy treatment photons, and these electrons can be affected by a magnetic field. In addition, the dose deposited in tissue can also be affected by a magnetic field.
Half of all cancer patients receive radiation therapy. (1) In recent years there have been tremendous advances in radiotherapy due to the use of image-guidance. However, the efficacy of image-guided radiation therapy (IGRT) for the treatment of thoracic and abdominal cancers remains limited due to geometric and dosimetric uncertainties caused by motion during dose delivery, where effective management of intrafraction motion is key to realizing the full potential of IGRT. Intrafraction motion management consists of two tasks: i) real-time estimation of tumor position and shape and, ii) corresponding real-time beam adaptation. Recent work in automatic detection of tumor volumes and adaptation of multi-leaf collimator systems during therapy addresses task two. However, with respect to task one, there is currently no method capable of directly visualizing a soft-tissue volume such as a prostate tumor or lung nodule during dose delivery. The most obvious choice is ultrasound, however soft tissue contrast is limited, and it cannot penetrate the ribs and air-filled lungs. Other options include projection radiography, which requires implantation of high-contrast beads in the tumor prior to treatment, or a calibrated monitor of external patient motion that correlates with internal tumor motion. Such approaches provide information of limited accuracy, reliability and reproducibility, and often increase interventional complications and imaging dose.
There is currently nothing available capable of directly visualizing a soft-tissue volume such as a prostate tumor or lung nodule during dose delivery. There are efforts in using linear accelerator-MRI to investigate ways to address the key issues, where the designs have the main magnetic field of the MRI scanner perpendicular to the radiation treatment beam. One group first mounts a low-field (0.2 T) permanent ‘C’ magnet and the Linac head on a rotating gantry with the RF source of the Linac in a separate room. Rotation is a significant engineering challenge. Another group has built a split-gradient 1.5 T system with the intent to rotate a linac around the MR system. The Linac will operate well at only one radial distance from the center of the magnet where the magnetic field is essentially zero. The major drawback of both geometries is that the therapy beam is at right angles to the main magnetic field B of the MR. The orthogonal geometry is sub-optimal for two reasons: first, the trajectories of electrons generated in the first few cm of tissue are affected by the presence of B, and deposit significantly higher skin dose than is desired; second, the electrons generated within the body next to air cavities are ‘bent back’ and again deposit undesired dose in adjacent tissue.
There are other groups attempting to integrate X-ray based radiation therapy and MR imaging, where only two are using an electron linac as an X-ray source. All of the above mentioned projects are using a perpendicular design characterized by the fact that the radiation beam is orthogonal to the magnetic field generated by the MRI system magnet. The main characteristic of a perpendicular design is that the dose distribution is going to be adversely affected by the existence of the magnetic field in the sense that there will be an extra dose deposited at the interface between air and tissue due to the return effect, which is going to affect healthy tissue. Also a perpendicular design, using a linac is more difficult to implement because one has to effectively isolate and decouple the operation of the linac and the MRI subsystems. It is clear that any transverse magnetic field to the accelerating structure will bend the electron beam due to the Lorentz force, which will result in no beam at the tungsten target.
Stereotactic body radiation therapy (SBRT) is used for early stage lung cancer. SBRT is a newly emerging radiotherapy treatment method to deliver a high dose of radiation to the target, utilizing either a single dose or a small number of fractions with a high degree of precision within the body. There are several trends in lung cancer, all leading to a sharp increase in the number of patients being treated with stereotactic body radiotherapy (SBRT). One of these is the increase in early stage lung cancer detection, and thus demand for treatment through CT screening programs.
Accordingly, there is a need to develop an apparatus that can provide real-time imaging of the position of a tumor while not interfering with the treatment beam, and a further need is an apparatus that can improve the quality of the treatment beam at the air-patient interface.